The invention is based on a fuel injection arrangement for internal combustion engines as defined hereinafter. In one such fuel injection arrangement, known from U.S. Pat. No. 4,721,247, a cam drive rotating in synchronism with the engine rpm drives an intermediate piston guided in a cylinder bore of a cylinder liner, and the piston in turn drives a pump piston, likewise guided in the cylinder bore, counter to the force of a restoring spring; a work chamber that can be filled with a hydraulic fluid is provided between the face ends facing one another of the intermediate piston and the pump piston. The pump piston is embodied as a stepped piston, whose larger face end defines the work chamber and which with its narrowed portion protrudes into a segment of reduced diameter of the cylinder bore, into the end of which remote from the pump piston injection ports to the combustion chamber of the engine to be supplied discharge, so that the cylinder bore is open toward the combustion chamber. On its narrowed end, the pump piston has a cross-sectional expansion, with which it defines a pump work chamber in the narrowed section of the cylinder bore; this chamber can be filled with fuel via a fuel supply line that can be opened by the jacket face of the pump piston, and this chamber communicates with the end of the cylinder bore that receives the injection ports. The face end of the narrowed part of the pump piston is also conically embodied and thus forms a conical sealing face, with which in the dead center position of the pump piston, after a maximum pumping stroke has been traversed, seals against a conical seat disposed in the cylinder liner and thus closes the communication with the injection ports.
The injection quantity is regulated by the degree of filling of the pump work chamber with fuel, while the instant of injection onset can be regulated by the filling of the work chamber with a medium (in this case fuel) acting as hydraulic fluid. The injection stroke of the pump piston is initiated by the closing of the supply line for the hydraulic fluid by the intermediate piston during its reciprocating motion, and as a consequence the enclosed pressure volume in the work chamber transmits the reciprocating motion of the intermediate piston hydraulically to the pump piston. The injection is terminated once the pump piston arrives with its conical sealing face at the seat in the cylinder bore and closes the communication with the injection ports into the engine combustion chamber. The pump piston in this position, with its larger face end toward the intermediate piston, controls a diversion bore of the work chamber, so that the hydraulic fluid can flow out as the stroke of the intermediate piston continues. The intermediate piston reaches its terminal position as a result of the contact with the larger face end of the pump piston, so that the latter, after the outflow of hydraulic fluid, is held mechanically by the intermediate piston in contact with its seat that closes the communication with the injection ports; this is necessary so that delayed injection of fuel into the engine combustion chamber will be reliably avoided.
The problem accordingly arises in the known fuel injection arrangement that the pump piston that closes the communication with the injection ports must be held mechanically on the seat in the cylinder bore at the end of the injection, which makes stringent requirements for adjustment tolerances necessary, while it can in turn be achieved only by difficult and expensive adjustment operations during the installation of the injection system in the engine. Moreover these adjustment tolerances can be strongly influenced by various operating parameters (temperature, wear), so that in the known fuel injection arrangement, the requisite adjustment accuracy for secure sealing over a long period of time only is attainable at very great effort and expense, and the danger of a major loss in efficiency from a leaking seat of the pump piston increases over a relatively long time in service.
Moreover, the fuel injection system, driven via a cam drive synchronous with the engine rpm, typically has the problem of an rpm-dependent high-pressure fuel pumping rate, where the maximum injection pressure is not available in all rpm ranges, and whose maximum pressure is no longer sufficient to meet the present high demands made of the injection pressure.